I'm having the exact same problem as NicolasDalby. If I use some older version (the one you can download from msgplus plugins section) it works, but if I update to the latest version, it doesn't. And yes, I have tried to uninstall/reinstall messenger, msgplus and file transfer plus. Nothing seems to help
I noticed that my messenger is not responding for some time after startup when the non-working plugin is installed, if that's of any help..